Optical recording media typically comprise an optical recording layer provided on a substrate. For media such as magneto optic recording media and WORM (write-once-read-many) optical recording media, the optical recording layer generally contains a thin film rare earth transition metal alloy such as gadolinium-cobalt (Gd-Co), gadolinium-iron (Gd-Fe), terbium-iron (Tb-Fe), dysprosium-iron (Dy-Fe), Gd-Tb-Fe, Tb-Dy-Fe, Tb-Fe-Co, terbium-iron-chromium (Tb-Fe-Cr), gadolinium-iron-bismuth (Gd-Fe-Bi), Gd-Co-Bi, gadolinium-iron-tin (Gd-Fe-Sn), Gd-Fe-Co, Gd-Co-Bi, or Gd-Dy-Fe. Such alloys are described, for example, in U.S. Pat. No. 4,822,675. For media such as compact disks, the optical recording layer may be a layer of reflective material, for example an aluminum or aluminum alloy, having a patterned, information-bearing surface.
Many of the materials which are suitable for the optical recording layer of optical disks react strongly with oxygen and other elements which may be present in the environment in which optical recording media are used. Furthermore, the substrate itself may contain impurities which react with the optical recording layer. Thus, transparent dielectric films may be deposited on one or both sides of the optical recording layer to protect it. Such dielectric films are described, for example, in U.S. Pat. Nos. 4,833,043 and 4,917,970.
Optionally, a reflective layer may be incorporated into optical recording media so that incident light that passes through the optical recording layer a first time, is reflected, and passes back through the optical recording layer a second time. Such reflection increases the magneto optic rotation of incident light because the so-called Faraday effect is added to the so-called Kerr effect.
The reflective layer may be incorporated into a magnetic recording medium such that the optical recording layer is interposed between the substrate and the reflective layer. For such media, transparent substrates are used so that incident light passes first through the substrate, then passes through the optical recording layer, and then is reflected by the reflective layer back through the optical recording layer. Such media are known as substrate incident media. Alternatively, when the optional reflective layer is disposed between the substrate and the optical recording layer, the read and write beams will not be directed through the substrate. Such a medium is known as an air incident medium, although generally there is at least one layer between the optical recording layer and the air.
For substrate incident media, the reflective layer is exposed to the environment, and therefore is subject to physical damage such as scratches, abrading, corrosion, and the like. For air incident media, the optical recording layer is exposed to the environment, and therefore is subject to physical damage such as scratches, abrading, corrosion, and the like. In either case, the optical recording layer or the reflective layer must be protected from such physical damage.
A "sealcoat" layer can be coated directly onto either the optical recording layer or the reflective layer to form a barrier protecting these layers from physical damage and the environment. Sealcoat compositions must satisfy stringent requirements in order to be suitable for use in optical recording media. Cured sealcoat layers should be abrasion resistant and compositionally stable so that the cured sealcoat layer maintains its protective properties for long periods of time. Preferably, a sealcoat layer will also protect an optical recording layer from corrosion.
The cured sealcoat is produced from an uncured admixture of ingredients which are first coated onto the optical recording media substrate, then cured. One method of applying sealcoats to an optical recording media substrate is by a spin coating technique. The spin coating technique requires that the viscosity of the uncured sealcoat composition be relatively low. For instance, to obtain a sealcoat layer of uniform thickness by the spin coating technique, the sealcoat composition preferably must have a viscosity of, for example, 100 centipoise or less at 25 C. The ingredients of conventional sealcoat compositions, however, may be of a relatively higher molecular weight and surface tension, causing admixtures of these ingredients to be too viscous to spin coat. One method by which the viscosity of these admixtures can be reduced is by adding a solvent to the admixture. Solvents, however, are generally disfavored because of their negative health and environmental effects, and attendant higher material and processing costs.